Mineralogic summary of Yucca Mountain, Nevada

Abstract

Quantitative x-ray powder diffraction analysis of tuffs and silicic lavas, using matrix-flushing techniques, has been used to obtain a model of three-dimensional mineral distributions at Yucca Mountain, Nevada. This method of analysis is especially useful in tuff, where the most abundant phases are commonly too fine grained for optical determination. The three-dimensional distributions of primary glass and of tridymite are particularly well constrained. Vitric nonwelded glasses occur above and below the welded devitrified Topopah Spring Member, but the glass in the lower nonwelded vitric zone is progressively altered to zeolites to the east where the zone is closer to the static water level. The zeolites clinoptilolite, mordenite, heulandite, and erionite have all been found at Yucca Mountain, but only mordenite and clinoptilolite are abundant and can be mapped between many drill holes and at many depths. Heulandite distribution is also mappa_,e, but only below the densely welded devitrified part of the Topopah Spring Member. Erionite has been confirmed only once, as a fracture coating. There is a fairly continuous smectite-rich interval immediately above the basal vitrophyre of the Topopah Spring Member, but no evidence suggests that the smectites can provide information on the paleogroundwater table. There are at least four mappable zeolitized zones in Yucca Mountain, and the thicker zones tend to coincide with intervals that retained glass following early tuff devitrification. Problems in extrapolation occur where zones of welding pinch out. No phillipsite has been found, and some samples previously reported to contain phillipsite or erionite were reexamined with negative results. The deeper alteration to albite and analcime was not sampled in every drill hole, and the distribution of these phases is difficult to map.

Distribution of potentially hazardous phases in the subsurface at Yucca Mountain, Nevada

“…As reported by Bish and Vaniman (1985), none of the previous reports of erionite in Yucca Mountain rocks has been verified. However, Bish and Vaniman reported that one sample of erionite has been conclusively identified in a fracture from uE-25a#1 at 1296 ft (395 m).…”

“…Los Alamos National Laboratory staff have worked to characterize and to understand the mineralogy and petrology of the rocks at and surrounding Yucca Mountain (Heiken and Bevier 1979;Sykes et al 1979;Bish 1981;Bish et al 1981;Caporuscio et al 1982;Vaniman et al 1984;Bish and Vaniman 1985). Bish and Chipera (1989) is the most complete and recent summary of the mineralogy of Yucca Mountain rocks obtained through quantitative X-ray powder diffraction analyses of drill core and cuttings.…”

Mineralogy of drill holes J-13, UE-25A No. 1, and USW G-1 at Yucca Mountain, Nevada

“…Using XRD, we have, however, recently identified erionite as a fracture-lining mineral in UE-25A#1-1296 (395.0 m) (Bish and Vaniman 1985).…”

Bulk, thermal, and mechanical properties of the Topopah Spring Member of the Paintbrush Tuff, Yucca Mountain, Nevada

“…Equation (6) may be used with porosity data for unit Tsw1 to compare calculated compressive strengths with those actually measured. Data in Bish and Vaniman (1985) has been analyzed to provide the following clay contents: 0.071 ± 0.040 in USW G-2 and 0.004 ± 0.005 in USW GU-3 (Note that a standard deviation greater than a mean value is indicative of a non-normal distribution of data). In addition, the average clay content of unit TSwl is 0.039 ± 0.041, based on x-ray analyses of samples from USW G-2, USW GU-3, and USW G-4.…”

“…Data in Bish and Vaniman (1985) indicate that the clay content of unit Tsw2 is 1.9~ ± 3.4~, based on X-ray analyses of samples from usw G-2, USW GU-3, and USW G-4. The clay is not uniformly distributed, being much more prevalent in USW G-2.…”